The invention relates to a circuit configuration for controlling a load with reduced noise emission. The circuit configuration has a switching device that is connected in series with the load between two supply potential terminals and a control device that actuates the switching device.
Circuit configurations of this type are commonly controlled in a pulse-width modulated manner in order to achieve current regulation in a load path. However, the pulse-shaped current consumption leads to undesirable noise emissions. In particular, the noise emissions can be disruptive to the functions of neighboring circuit configurations. The noise emission (EMV-noise) is mainly produced when the switching device that is connected in series with the load is switched completely on or completely off. The EMV disturbances are caused specifically by those regions of the current characteristics that contain a particularly abrupt relative decrease or increase of the current flow. During the rising edge operation, the current increase from zero to a value other than zero is considered particularly critical. However, the disrupting radiation is prevented when the current characteristic assumes a sinusoidal course. Given pulse-width modulation, the noise spectrum is determined by the shape of rising and falling edges. In particular, the amount of noise emitted is determined by the edge steepness of the current characteristic. The flatter the edge steepness, the less noise emission. But a flat current edge has the disadvantageous result that it drastically elevates the switching losses. Therefore, to prevent switching losses and thus a thermal heating of the semiconductor switch, it is expedient to configure the current edges as steep as possible on both the leading and trailing sides. On the other hand, this causes the electromagnetic noise emission to increase.
When MOS switches are used as semiconductor switches, the lower corners of the rising and falling edges are particularly critical with respect to noise emission.
For these reasons, efforts have been made to find an optimal compromise between the power loss of the semiconductor switch and the generated noise radiation. In the prior art, it is proposed that this object is achieved by performing a control or regulation of the gate voltage of the power transistor. Besides a complicated control circuit, the proposed solution has the disadvantage that only the falling edge is affected; that is, it is rounded off. In addition, a higher circuitry outlay is required for the control circuit, which requires additional space in an integrated circuit configuration and is thus expensive, and which still causes EMV disturbances during the rising edge.
It is accordingly an object of the invention to provide a circuit configuration for controlling a load with reduced noise emission which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which configuration causes significantly less noise.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for controlling a load and producing reduced noise emissions. The circuit configuration contains two supply potential terminals including a first supply potential terminal and a second supply potential terminal. A switching device is provided for connecting in series with the load between the two supply potential terminals. The switching device contains semiconductor switches including a first semiconductor switch and a second semiconductor switch. The semiconductor switches have load paths connected in parallel and each of the semiconductor switches has a threshold voltage. The threshold voltage of the first semiconductor switch is higher than the threshold voltage of the second semiconductor switch. A control device is connected to and controls the switching device.
In accordance with an added feature of the invention, the semiconductor switches are field-effect-controlled components.
The invention is based on the recognition that the noise emission that is caused by a semiconductor switch can be reduced by rounding off the rising and falling edges of the current characteristic. This is achieved in that the switching device contains a first and at least one second semiconductor switch, whose load paths are connected in parallel, the threshold voltage of the first semiconductor switch being larger than that of the second semiconductor switch. The threshold voltages of the semiconductor switches can be defined either by differences in the doping of their bulk regions or by different thicknesses of their gate oxides.
During a rising edge, the second semiconductor switch with the lower threshold voltage thus starts to conduct current even when given smaller control voltages by the control device. Therefore, this semiconductor starts to conduct current sooner. A short time later, the first semiconductor switch, which has the higher threshold voltage, also conducts. In a corresponding manner, during the falling edge the second semiconductor switch (having the smaller threshold voltage) remains conductive longer than the first semiconductor switch. Due to the superimposing of the currents of the first and second semiconductor switches, the lower corners of the switching edges in the rising and falling processes are rounded off. As a result, fewer harmonic components arise in the noise spectrum.
In an advantageous development, the control terminals of the first and second semiconductor switches are connected to one another. The result of this is that all three terminalsxe2x80x94the control terminal and the two main terminalsxe2x80x94of the first and second semiconductor switches are connected to one another. The switching device is thus actuated via a single actuation by the control device. The different switching behaviorsxe2x80x94that is, the instants of the conducting and non-conducting of the respective semiconductor circuitxe2x80x94are thus determined exclusively by the threshold voltages of the first and second semiconductor switches.
It is also advantageous to equip the first semiconductor switch with a larger number of cells than the second semiconductor switch. The number of cells of the second semiconductor switch preferably amounts to between two and five percent of the number of cells of the first semiconductor switch.
Alternatively, the first semiconductor switch contains a significantly higher W/L ratio than the second semiconductor switch. W represents the channel width, and L represents the channel length of a field-effect-controlled component.
The inventive circuit configuration can either be constructed in a monolithic integrated fashion or can consist of discrete components. The extremely simple structure of the circuit configuration is suitable for discretely constructed semiconductor components, since these usually do not have separate logic circuits. For this reason, it has been necessary hitherto to provide additional external components in order to influence the edge control, such as components for appropriately controlling or regulating the gate voltage.
The inventive circuit configuration is characterized namely by its extremely simple construction and by the possibility of forgoing any sort of control or regulation. A very economical production is possible, since additional space on the semiconductor chip is not needed given an integrated construction. In most modern integrated technologies, cells having different threshold voltages are available. Thus, manufacturing can be realized using the known production technologies without additional steps or masks. The invention advantageously affects the rising and falling edges of the current characteristic in equal measure. It can be applied in both high-side and low-side configurations.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for controlling a load with reduced noise emission, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.